/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> vv;
        if (root == nullptr)
            return vv;

        queue<TreeNode*> q;
        q.push(root);
        int levelSize = q.size();
        while (!q.empty())
        {
            vector<int> v;
            while (levelSize--)
            {
                TreeNode* node = q.front();
                v.push_back(node->val);

                if (node->left)
                    q.push(node->left);

                if (node->right)
                    q.push(node->right);

                q.pop();
            }

            vv.push_back(v);
            levelSize = q.size();
        }

        return vv;
    }
};